A major world economic power, Japan is a leading Annex I member of Kyoto, committed to cutting emissions. It feels an attachment to the protocol, named after the Japanese city where it was concluded. It recognises the argument that its economy could gain from seeing the treaty in force, as Japanese companies could capture markets for new, clean technology.
But Japan is very reluctant to ratify Kyoto unless the Americans do so as well. Without Japanese ratification the protocol is very unlikely to attract the support it needs to become international law.